[99-LS1-SS]

IF I'm not mistaken the 4L60E is based off of the TH350 and the 4L80E is based off of the TH400. If strength is a consideration I would go with a TH400 if mileage is the consideration then the 4L60E.
This is just MY personal opinion so take it for what it is worth. I have never been impressed with the strength or durability of the 4L60E. If it was up to me I would go with the 4L80E. It will handle more power and abuse and it should last longer.